1. Field of the Invention
In a general aspect, the present invention relates to a superconducting cable comprising a cryogenic fluid, a superconducting conductor and a cryostat.
More particularly, the invention relates to a superconducting cable comprising a cryogenic fluid, a superconducting conductor and a cryostat wherein the cryogenic fluid cannot reach a direct contact with the superconducting material.
2. Description of the Related Art
The term xe2x80x9csuperconducting cablexe2x80x9d encompasses any cable to be used to transmit current in conditions of so-called superconductivity, i.e. in conditions of almost null electric resistance. See, for example, Engelhardt J. S. et al., Application Consideration for HTSC Power Transmission Cable, 5th Annual Conference on Superconductivity and Application, Buffalo, N.Y., Sep. 24-26, 1991.
The term xe2x80x9csuperconducting conductorxe2x80x9d indicates in the following any element capable of transmitting electric current in superconductivity conditions. An example would include a layer of superconducting material supported by a tubular core. Another example would include tapes of superconducting material partially or totally surrounded by a noble metal pipe, which are wound on a supporting core.
The term xe2x80x9csuperconducting materialxe2x80x9d or xe2x80x9csuperconductorxe2x80x9d indicates a material such as, for example, special ceramics based on mixed oxides of copper, barium, and yttrium (usually called YBCO); of bismuth, lead, strontium, calcium, and copper (usually called BSCCO); or of thallium or mercury and barium, calcium, and copper, comprising a superconducting phase having a substantially-null resistivity under a given temperature, defined as the critical temperature or Tc. For example, for the above-mentioned materials, the Tc ranges from about 80 K (xe2x88x92193xc2x0 C. to about 150 K (xe2x88x92123xc2x0 C).
Usually, the superconducting material, particularly the BSCCO material, is produced and used in the form of mono- or multi-superconductor element tapes. The material is surrounded by a metal, generally silver, optionally with aluminum or magnesium added.
The operative temperature of a superconductive cable is lower than the Tc of the superconductive material present therein.
In view of this the superconducting cables are provided with at least one channel for the flow of the cryogen. The cryogen is typically liquid helium, liquid nitrogen, liquid hydrogen and/or liquid argon, operating at temperature and pressure specific for the application.
The term xe2x80x9coperative temperaturexe2x80x9d indicates in the following the temperature at which the superconducting cable transmit electric current in superconductivity conditions. Specifically such temperature is lower than the Tc.
For maintaining the superconducting material at the operative temperature a close contact between the superconducting material and the cryogen is generally recommended. See, for example, EP-A-0 786 783 (in the Applicant""s name) wherein the cryogen is said to flow both in the inside of the conductive elements and in the interstices between such elements and the tubular shell. U.S. Pat. No. 4,966,886 (in the name of Junkosha Co., Ltd) discloses a cable wherein the liquid nitrogen penetrates into the crystalline arrangement of the superconducting ceramic and is effectively absorbed to yield a stabilised superconducting cable. Analogously, EP-A-0 412 442 (in the name of Sumitomo Electric Industries, Ltd.) discloses a pipe supporting the superconducting tapes and defining the cooling space, said pipe being provided with holes in order to increase the efficiency of the cooling.
EP-B-0 297 061 (in the name of Saes Getters S.p.A.) discloses a vacuum insulated superconducting electrical conductor employing a getter device. More specifically, this document claims an electrical conductor wherein a thermally insulating evacuated space surrounds the superconducting elements and the liquid nitrogen. Said evacuated space takes the place of the cryostat which is absent in the cable described in the above patent.
WO 98/09004 (in the name of American Superconductor Corporation) discloses that the infiltration of cryogenic liquid into the porous ceramic structure of the superconducting material is detrimental for the integrity of the conductor. In fact, when the article is rapidly heated, the cryogenic liquid entrapped in the interstices of the ceramic material quickly expands, thus creating xe2x80x9cballoonsxe2x80x9d in the matrix and damaging the intragrain bonds thereof. This causes a decrease of the mechanical strength and current carrying capacity of the article. According to said patent application, it is known to xe2x80x9cpotxe2x80x9d certain superconducting articles with thick layers of epoxy resin for minimising the likelihood of contact between the liquid and the superconducting tape. Alternatively, when the article cannot be protected in such a way (the use of a heavy epoxy coating is considered not feasible because of a number of reasons such as packing factor and flexibility requirements) other cooling means, such as conductive cooling are used. Nevertheless, it is said that the cooling by conduction is not deemed adequate for applications such as transmitting cables. This patent application proposes to solve the xe2x80x9cballoonxe2x80x9d problem by a superconducting conductor wherein the superconducting ceramic tape has at least one surface, which is vulnerable to cryogenic infiltration, sealed to a non-porous metal laminate impervious to said infiltration. In particular, the metal is stainless steel, copper, copper alloy, or superalloys.
The Applicant has found that the xe2x80x9cballoonxe2x80x9d phenomenon does not only occur due to cryogenic fluid leaking from the flowing channel into the superconducting tape area. Actually any kind of fluid directly in contact with the superconductor may liquefy at a temperature equal or higher than the operative temperature of the cable and penetrate into the superconductor. When the temperature of the article rapidly increases, for example when the cable is brought to room temperature for maintenance operations, such a liquefied fluid will abruptly turn into gaseous status, thus expanding its volume and consequently damaging the superconductor according to the xe2x80x9cballoonxe2x80x9d effect discussed above.
Moreover, in the Applicant""s view, the prior art technique of individually protecting each tape to prevent the xe2x80x9cballoonxe2x80x9d formation, which implies the production of superconducting cables provided with this specific kind of tapes, is economically inconvenient as further material (stainless steel, copper, copper alloy, or superalloys) and further processing steps (lamination and sealing) are necessary.
It has been found that the xe2x80x9cballoonxe2x80x9d effect damaging the superconducting material can be effectively eliminated by providing a layer of material impervious to the cryogenic fluid. The layer of material can be added between the superconducting conductor and the fluid flow. This addition causes the superconducting material to operate in a space free from fluids that liquefy at a temperature equal to or higher than the operative temperature of the superconducting material.
Therefore, the present invention relates to a superconducting cable that comprises a cryogenic fluid, a superconducting conductor, and a cryostat. Further, a layer impervious to the cryogenic fluid is provided between the superconducting conductor and the cryogenic fluid. This permits the superconducting conductor to operate in a space substantially free from fluids that liquefy at a temperature equal to or higher than the operative temperature of the superconducting cable.
The superconducting cable of the present invention may be a warm dielectric (WD) or cold dielectric (CD) cable. See, for example, Engelhardt J. S. et al. supra, FIG. 5 for a WD cable, and FIG. 6 for a CD cable.
A WD cable generally comprises superconducting tapes wound on a support, typically tubular, defining the cryogen fluid flow channel. Externally to the superconducting tapes a cryostat and an electric insulation are provided.
A CD cable generally comprises, in addition to the conductor mentioned above for the WD cable, a further superconducting conductor, called return conductor, wound externally to the electric insulation and surrounded by a layer partially defining a second cryogen fluid flow channel.
In the case of the CD cable, both of the superconducting phases may be contacted by said impervious material in the direction of the cryogenic fluid, and both of them may be in a space free from fluids liquefying at a temperature equal or higher than the operative temperature.
The layer impervious to the cryogenic fluid, provided between the super-conducting conductor and the cryogenic fluid is made from a material having a thermal conductivity so as to allow a thermal flow between the cryogenic fluid and the superconducting conductor sufficient to achieve an effective cooling of the superconducting material when the cable operates. Preferably, said material has a thermal conductivity higher than 1 W/m K at 70 K, even more preferably equal or higher than 2 W/m K.
The impervious material, which constitutes per se a barrier to the cryogenic fluid, may be of a metal such as, for example, copper, steel or aluminium, or of a polymeric substance such as a fluorinated polyolefin (e.g. polytetrafluoroethylene), a polyolefin (e.g. polyethylene), a polyamide (e.g. nylon) and the like.
The impervious layer may be in form of a tube, of a spiral contained in a metal tube, or of a plurality of adjacent tape-shaped elements spirally wound to form a tube and contained in a tube. Said tape-shaped elements may be made of the same or different materials, e.g. copper alternated by polytetrafluoroethylene.
The cooling performance of the cryogenic fluid depends not only on the thermal conductivity of the impervious layer, but also on its dimensions (for example, on the thickness and/or the diameter thereof, and, in the case, on the number of the superconducting tapes to be cooled.
Preferably, the inner diameter of the impervious layer is comprised between about 10 and about 50 mm, while the thickness is determined according to both the material (metal and/or polymer) and the kind of cable (WD or CD) and also, optionally, the number of overlapping layers of superconducting tapes.
When more layers of superconducting tapes are provided in the cable of the invention, it takes to evaluate the difference of temperatures (xcex94T) between the first layer, i.e. the nearest to the impervious layer, and the last one.
For example, 1 km of a cable comprising two layers of 24 BSCCO tapes each having a thickness of 0.3 mm wound on a copper tube as support and impervious layer having a thickness of 3.5 mm and an internal diameter or 38.7 mm, and carrying 2,600 A of A/C current was cooled with liquid nitrogen at 65 K at the inlet, resulting in a liquid nitrogen temperature of 84 K at the outlet.
The cryogenic fluid useful for the cable of the present invention may be any fluid having, at the operative condition, a transition temperature from liquid to gaseous state higher than the Tc of the superconducting material, i.e. liquid helium, liquid nitrogen, liquid hydrogen and/or liquid argon. Preferably, the cable of the invention is cooled with liquid nitrogen at a temperature typically of from about 65 to about 90 K.
The superconducting material of the cable of the present invention may be, for example, an oxide of lanthanum and/or barium and/or strontium, copper (LaSCO) or of bismuth, lead, strontium, calcium, copper (BSCCO), or of yttrium (and/or other rare earth such as Nd, Sm, Eu, Gd), barium, copper (YBCO), or of thallium, barium (and/or strontium), calcium, copper, or of mercury, barium (and/or strontium), calcium, copper, or of lead, strontium, yttrium, copper.
The superconducting material of the invention may be produced by any of the methods known in the art, for example, by the oxide powder in tube (OPIT) method for the BSCCO, or by that described in U.S. Pat. No. 5,741,377 (in the name of Martin Marietta Corporation) for YBCO.
The tapes may also be mono- or multi-superconductor elements, as described in application EP 0,747,975 (in the Applicant""s name).
The space free from fluids that liquefy at a temperature equal to or higher than the operative temperature provided for the superconducting cable of the present invention may be under a vacuum. Alternatively, an atmosphere of a fluid, that does not liquefy at a temperature equal to or higher than the operative temperature of the cable (e.g., a helium atmosphere), can be used for a cable having nitrogen as the cryogenic fluid. Preferably, the superconducting conductor is under vacuum. In this case, the superconducting area is preferably provided with getters, i.e., gas absorbers of sintered powder of, e.g., zirconium or titanium. See, for example, della Porta, P., xe2x80x9cGetteringxe2x80x94an Integral Part of Vacuum Technologyxe2x80x9d, American Vacuum Society, 39th National Symposium (Technical paper TP 202).
The method for making the vacuum are those known to the skilled in that art, for example by vacuum pumps. The vacuum has a value of at least 10xe2x88x924 bar, preferably 10xe2x88x927 bar.
The cryogenic fluid remains confined into its flowing channel(s) and does not get in direct contact with the superconducting conductor. At the same time the superconducting material is anyway efficiently cooled.
In another aspect, the present invention relates to a method for protecting a superconducting cable, comprising a cryogenic fluid, a superconducting conductor, and a cryostat, from the formation of balloons. The cryostat isolates the superconducting conductor from the cryogenic fluid by a layer impervious to the cryogenic fluid. Furthermore, the superconducting conductor operates in a space free from fluids that liquefy at a temperature equal to or higher than the operative temperature of the cable.
According to a further aspect, the invention relates to a current transmission/distribution network comprising at least one superconducting cable comprising a cryogenic fluid, a superconducting conductor, and a cryostat. The cryostat includes a layer impervious to the cryogenic fluid between the superconducting conductor and the cryogenic fluid. The superconducting conductor is operated in a space free from fluids that liquefy at a temperature equal to or higher than the operative temperature of the conductor.